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vent and the substance dissolved." In describing the
present state of our knowledge, he brought forward some
fads which have a bearing on the views under discussion.

In the first place, although it may be accepted as a rule
that a solid when dissolved retains its former refradive
power, it is a lule not without exceptions. Thus the ex-
periments, both of the speaker and of Dr. Bedson, on rock
salt agree in giving 14*6 as ihe molecular refradion of
chloride of sodium for the solar line A or r ; in which r

II— 1
represents the value ^ multiplied by the molecular

weight. But the molecular refradion for the same ray as
calculated from aqueous solution is 15*3, showing that the
water has perceptibly increased the refradive power. And
this is not an isolated instance, for the observations of
Topsoe and Christiansen on crystals of potassic bromide
and iodide show a molecular refiadion for the line d, or
Rd, of 24*85 and 36*29 respedivtly, while the solutions
indicate 257 and 36*9 respedively. In fad, the chlorides,
bromides, and icd:des, in general, when dissolved in water,
are known to exhibit a higher refradion and dispersion
than would be calculated by adding together the generally
received values for the metal and the halogen, and this
increase is uniform for each series of salts.

It is also known that there is a slight change in the
molecular refradion of certain liquid substances, such as
acetic acid, when they are mixed with water.

In the second place the molecular refradion of a sub-
stance in solution is not varied by varying the amount of
the Folvent. In the case of ^^ater, however, there are
some marked exceptions. With the hydracids the values
increase with the dilution up to a certain extent, when
they become stationary. Nitric and sulphuric acids are

* Britifcb Association (Scdioo B), Leeds Meeting, 1^90.



also exceptional. It is evident that the difiPerence here
noted does not depend upon whether these binary com-
pounds are eledrolytes or not.

In the third place there is a great deal of evidence that
the molecular refradion of a substance is the same whether
it be deduced from its solution in alcohol, ether, benzene,
bisulphide of carbon, or any other solvent that does not
ad chemically upon it. The same rule applies in some
instances to solution in water ; thus the molecular refrac-
tion of ammonia in alcohol, or in different quantities of
water, was found to be about 8*96. The value for gaseous
ammonia, as deduced from Oolong's observations, is 8*60.

A notable exception is hydrochloric acid. Very early
in the history of refradion equivalents it was recognised
that this acid in aqueous solution gave a value much
larger than the gas itself, or than what would be obtained
by adding together the values for chlorine and hydrogen
in combination, as deduced from other sources. Dr..
Perkin found a similar great increase of magnetic rotation
in an aqueous solution of hydrochloric acid, but on dis-
solving the gas in Isoamyloxide and examining the solu-
tion he found it rotated the plane of polarisation to very
little more than the theoretical amount. The speaker
therefore determined the refradion of this solution, and
found the hydrochloric acid in it to have pradically the
theoretical value.

HCl, theoretical value .. •• 11*2 or 11*3

HCl, in water about 14-4

HCl, in isoamyloxide .. .. 11*36

It would not be safe to use this increase of refradion
of hydrochloric acid in aqueous solution as an
evidence either of dissociation or of the formation of a
hydrate. For the sum of the molecular refradions of free
hydrogen and free chlorine, as determined by Dulong or
Mascart, would be only 10*3, rather less than the theo-
retical, instead of more, as might beexpededon the disso-
ciation hypothesis,* while, on the other hand, the addition
value of HaO in recognised hydrates (such as crystallised
alums) seems to be the same as that of pure water,
namely, 5*93.

The general inference drawn by the speaker from the
accumulated evidence was that the old conclusion is sub-
stantially corred ; that molecular refradion and dispersion
may be sa'ely deduced trom substances in solution where
the solvent is chemically inactive, but that in the case of
water there is some profound change effeded upon the
constitution of hydracids, haloid salts, and probably some
other compounds by the ad of solutipn. What this change
may be cannot at present be inferred from optical
analysis.

Dr. James Walker read the following translation of a
communication from Dr. Arrhenius: —

•* In the yourn, Chem. Soc. for 1890, p. 355, Mr.
Pickering writes :— * It is indeed surprising that van 't
HofT, Arrhenius, and others should not have recognised
that every known deviation from the so-called normal
depression, when induced by increase of strength of the
solution, is in exadly the opposite diredion to that which
it should be if the law of osmotic pressure were really
corred.* That tlie c'epress'on of the freezing point per
grm. molecule should decrease with increasing concen-
tration is no dedudion (as Mr. Pickering seems to
imagine) from the law of osmotic pressure ; and the
corresponding statement for the analogous case of highly
compressed gases has been proved to be false by the re-
searches of Rcgnault, Netterer, and others. . . . Be-
sides, it is not corred that ' every known deviation ' is in
the opposite diredion to that expeded by Mr. Pickering.

* These number* i^-culd have been brought mote closely together
if the calcu'aticD htd bern made by means of Lorenx't formuJai

X J instead of the simpler ^—

/ii^+a d

it is pradically cnimportant which formula is employed.



With liquids andsoHds



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148






Theory


of Solution.






i March 26. 1891.
















Differences in i-xooo


BiSO« per cent


Sp.Rr.


Qn-eqniv.


<.-i+atf


Freeziog-point.




deg.<


Cent.


Obi.t


Obs,


Calc.


Obi i-calc.


ObB.a-calc.


3*993


1*0278


0-8376


2-036


-x-6x'»





-1-62


~I0





3-967


1*0274


0-8324


2-036


x-58





161


-30





3-492


XO243


0-7300


2-042


x-37





1-42


-50





3*oo8


Z-0210


0-6267


2-051


I-X9





1*22


-30





2806


X0193


0-5835


2-058


x-io





1*14


-40





2-496


1-0174


0-5182


2*064


0*981





1-016


-35





1-996


Z'0Z40


0-4130


2-082


0-788





0*817


-29





1785


Z'OZ26


0-3688


2-088


0705


0-699


0-731


-26


-32


1-596


Z*0ZI2


0*3293


2-094


0*633


0-627


0*655


-22


-28


1-398


Z'OIOO


0*2882


2x02


0558


0-550


0-575


-17


-25


Z'2I2


1*0087


0-2496


2-XX2


0484


0-480


0*501


-17


-21


1*024


1*0073


0*2058


2-X26


0-417


0*412


0*416


+ 1


-4


o-8z88


1-0059


0*1681


2-X36


0-334


0332


0-333


+ 1


-I


0-7138


1*0051


0-1464


2*146


0297


0-294


0*298


— 1


-4


0-6145


x-0044


0*1260


2-X56


0*255


0-254


0*258


-3


-4


0-5146


1*0037


0*1054


2-168


0-219


0*217


0*217


+2





0*4061


z'0029


,0*083x2


2-2IO


0-178


0*177


0*175


+3


+2


0-3562


1*0025


0-07288


2236


0-160


0-155


0-155


+5





0*3063


Z-0022


0*06264


2*272


0*138


0137


0-135


+3


+2


0-2594
0*2056


Z*00I9


o*0528x


2*304


0-1x5


o*ii6


o*xi6


— 1





z*ooi5


0-04203


2*352


0-095


0093


0-094


+ 1


-I


0*1539


1*001 X


0*03144


2-406


0*067


0*072


0-072


-5





0-Z40Z


I'OOXO


0*02861


2*422


0-062


0*067


0-066


-4


+ 1


0*ZOI2


1*0007


0*02067


2-476


0052


0-049


0-049


+3





0*0771


z*ooo5


0*01574


2-544


0-038


0-035


0-038





-3


00519


z*ooo3


0*0x060


2594


0*026


0-028


0-026





+2


0*0264


1*0001


0-00539


2-702


0*016


0014


0*014


+2

















Sum


•• +7


-10



From Beckmaon*8 excellent determinations (Zeitsch.fttr
Pkysih, Ch€m,f ii., 715) it appears that in the great
majority of cases the molecular depression does diminish
with increasing concentration when benzene and acetic
acid are the solvents. Mr. Pickering can find numerous
other examples in Eykman*s observations, and I shall
show below that it is even the case with the sulphuric
acid solutions which were the subjed of his own investiga-
tion. • . •

'* Mr. Pickering, in comparing his ' theoretical ' with
the observed values for the depression of the freezing-
point in dilute solution of sulphuric acid, remarks that
' the molecular depression, iV€H in this extreme region,*
instead of being constant, as it should be according to
the theory of osmotic pressure, varies between 2*95° and
2*1*.' Mr. Pickering has overlooked the fad that sul-
phuric acid is an eledrolyte, and that the deviations may
oe accounted for by the theory o( eledrolytic dissociation.
For the purpose of comparison with the experimental
results, I have calculated the values of the depression for
dilute solutions, such as Mr. Pickering investigated. In
the calculation I have taken the freezing-point of an
aqueous solution of a non- eledrolyte containing 1 grm.
molecule per litre to be — x*9o* C, in accordance with
van *t Hotrs theory. I have further made the molecular
condudively of iH2S04 at infinition dilute Cuw) equal to
356/10' Siemens*s units fKohlrausch, Wied, Ann., xxvi.,
196). From Kohlrausch's numbers we then find the
degree of dissociation—*

a for X 0-5 o*x 0-05 0-03 o*oi 0*006 0-002*

to be 0*511 0*533 0-585 0*658 0-707 0*802 0-844 0*9x0

* Normal tfilntloni.

By interpolation we get a for other concentrations
{Zeitseh,f, physik, Chem,, v., 5). From the percentage
composition and the specific gravity (Pickering) I have
calculated the number of grm. -equivalents per litre solu-
tion. The subjoined table corresponds to that on p. 363
of the your, Chem, Soe.

Mr. ?.*• italics.



'* Under obs.x are the (correded) observed numbers
obtained with thermometer 65108; under obs.a are the
numbers for the same concentrations interpolated from
the series made with themometer 65561. This compari-
son affords an indication of the experimental accuracy.

'* It is at once evident from the table that the observed
numbers agree within the limits of experimental error
(obs.x— obs.a) with the theoretical values so long as the
concentration is less than 1 per cent. The agreement, in
fad, is so extremely good as to lead one to put more faith
in the calculated than in the observed values. In stronger
solutions (i to 4 per cent) the depressions found are less
than the theoretical depressions, in dired contradidion to
Mr. Pickering's statement that the opposite is always the
case. On this last circumstance, however, we need not lay
too much weight, for the theory has not yet been suffi-
ciently advanced in this diredion, and the deviations
besides only amount here to 3*6 per cent at most. In-
stead, then, of these experiments of Mr Pickering finally
disproving * all existing physical (sic/) theories of solu-
tion,* and in especial ' the theory of osmotic pressure,*
they afford the most striking proof of the applicability of
van 't Hoffs theory and the hypothesis of eledrolytic
dissociation of dilute solutions, with which alone these
theories have hitherto been concerned."

Dr. Walker drew attention to the fad that in almost
all the combinations of solvent and dissolved substance
tabulated eledrolytic dissociation played a great part,
entirely negleded by Mr. Pickering. The comparison of
observed with ** theoretical *' values was thus open to the
same objedion as Dr. Arrhenius urged in the case of
dilute solutions of sulphuric acid, and so the great dis-
crepancies found in the tables were from this cause alone
rendered illusory.

Professor Ramsay suggested that it might well be the
case that complex molecular aggregates were capable of
existence alongside of dissociated molecules where ions
are present. In the case of solutions of sulphuric acid,
for example, it is by no means unconceivable that aggre^
gates of several molecules of sulphuric acid (HaS04)fi, or
of compounds of acid and water, such as HaS04.2H20,



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CsBHtCAt NBWi \

March 20, X89X. I



Theory of Solution.



14^



&c., might exist along with the ions of dissociated sul-
phuric acid aH and SO^, or more probably H and HSO4.
The abnormal results in the freezing-points of solutions
of sulphuric acid observed by Mr. Pickering might well
be due to some such cause.

Dr. Armstrono, after remarking that thus far the
physical aspens of the main problem under discussion —
the constitution of solutions which conduced eledlrolyti-
cally — had alone been dwelt on, said that it would be
impossible, in the time at disposal, to consider more than
one of the conclusions arrived at by the advocates of the
dissociation hypothesis, which did not appear to be in
accordance with the chemist^s experience. It had hitherto
been customary to regard the neutralisation of an acid
by an alkali as a case of interchange or double decom-
position, as represented, for example, by an equation such
as KOH + HCl » KCl + HOH. But now that it was
argued that hydrogen chloride, potassium hydroxide, and
potassium chloride underwent almost complete dissocia-
tion when dissolved in water to form a dilute solution, it
became necessary to suppose that in such cases the only
new compound formed m solution was water, and the
main adion which occurred on mixing solutions of
potassium hydroxide and hydrogen chloride was conse-
quently represented by the equation

H+Cl+K+OH = K+Cl + HaO.

Such a conclusion, although undoubtedly a necessary
and logical one from the dissociationist*8 point of view,
involved the admission that hydrogen chloride and water
were compounds of a totally different order ; that these
two hydrides were so different that while that of chlorine
underwent pradically complete dissociation that of
oxygen remained pradically unchanged. Chemists, how-
ever, were in the habit of teaching that chlorine and
oxygen were comparable elements, and the fads of
chemistry appeared to afford the strongest evidence that
hydrogen chloride and oxide were in all ways comparable
compounds. Moreover the behaviour of the two com-
pounds at high temperatures afforded no grounds for any
such belief in the instability of the one and the stability
of the other.

Referring to the series of numerical agreements
between theory and pradice relied on by the dissociation-
ists, the speaker said that in his opinion these afforded
no necessary proof of the corredness of the theory. The
correlation of chemical adivity and eledrical resistance
which had been established by Arrhenius, Ostwald, and
others was undoubtedly of the highest importance, but
the successful use which they had made of the data at
their disposal appeared -to him to depend on the fad that
by observations of eledrical resistance they were enabled
to classify eledrolytes in the order of their adivity,
whether physical or chemical; and that, having done
this, they were in a position to apply the corredion
required to discount the superior adivity of such com-
pounds in comparison with dieledrics, f.^., compounds
producing the so-called normal effed in depressing the
freezing-point, for example.

Professor Fitzobrald said—It is important to dis«
tinguish between what is implied and what not by experi-
ments: #.^., osmotic pressure, change of freezing and
boiling points are in no way independent ; we can deduce
one from the other by applying known principles. There
seems to be a very important connedion, which cannot
be deduced from known principles, between condudivity,
the variation of osmotic pressure from its value calculated
from molecular weights, and the chemical adivity of a
substance in certain relations. The quality upon which
these properties depend is, I think, certainly the same
quality in each case, and its existence and importance
have been brou|[ht to light by the labours of our renowned
visitors and their collaboratenrs, and the discovery is one
of the most valuable contributions to chemical physics
that has been made of recent years. The visitors call
this quality the *' ratio of disaociation." Professor Ann<



strong would rather call it ** measure of affinity.'* I would
be inclined to point out that the term ** dissociation'* is
not happily chosen, and that ** affinity*' really explains
very little, and that it would be better to call it by a new
name whose full meaning will require further investiga-
tion, and would call it ** measure of ionisation.'*

In the first place as to the term ** dissociation.** In all
other cases of dissociation, e.g. in an eledric arc, the
elements are so far free from one another that they diffuse
independently of one another. The term *• dissociation*'
is no doubt vague, but it is time we had a more definite
notion of it. 1 would certainly confine the use of the
term to such cases that there was no link conneding the
elements that would prevent their diffusing independently
of one another. As long as there is any link conneding
the elements of molecules together which essentially pre-
vented one of them getting away without the others fol-
lowing, I would iTot agree to say that the elements
were dissociated. Hence I objed to the term dissociation
as applied to the ions in an eledrolyte. All agree that
one cannot escape or diffuse without^he other following ;
it may be due to eledrical forces between them, it may
be for other causes ; but in either case I would refuse to
call them dissociated. The possibility of independent
diffusion I look upon as a test of dissociation. I would
therefore appeal to both sides to adopt some neutral term
such as ** ionisation'* to express the state of ions in elec-
trolytes. Now as to the proofs that the ions are abso-
lutely independently mobile in the liquid, and the
assumption from this that they are free like the molecules
of a gas being kept apart by the molecules of the solvent.
This seems a very misleading way to speak of the condi*
tion. In the first place it is acknowledged that different
solvents have different powers of ionising a given sub-
stance, thereby conclusively proving that the fundion of
the solvent cannot be properly described as merely giving
the ions space to resolve themselves. And those who
speak so acknowledge that it is only an analogy, or a
fa^n de parUr, But it seems a very misleading analogy,
which leaves out the really active part that the solvent
plajrs, and attributes to it a purely passive part. The
argument of vnn 't Hoffthat the osmotic pressure in very
dilute solutions depends only on the kinetic pressure, and
not on the forces between the molecules, seems to cut
against the conclusion that these forces must neces-
sarily be small; it seems to show that, whaUvtr
forces there are between the ions, they will pro-
duce the right amount of osmotic pressure if
only they are so far independent that each ion can
carry on its bombardment independently of the other.
As this only requires the space within which they are
bombarding about to be small compared with the space
rate of variation of the force between the elements, and
as this is quite consistent with there being plenty of
connedion between the elements, it follows that the laws
of osmotic pressure so explained do not in the least mili-
tate against there being bonds between the elements.
The whole argument is, however, I think, fallacious, in
that it assumes a particular theory as to the adion
between the semi-permeable membrane and the liquid. It
would follow from this theory that one molecule of a salt
could never produce osmotic pressure in its own neigh-
bourhood by any forces of attradion between it and the
solvent. Now if we apply this on a large scale to the
case of an ocean zooo miles deep surrounding the world
with a membrane in it, say, zoo miles deep, through
which the water could go, but the world could not because
the holes were only, say, about a square mile in area, we
see at once that, if this membrane were made of a mate-
rial lighter than water, i.f., less attraded by the world .
than water, it would tend to burst out with a great force,
i.tf., it would float out from the world because the pres-
sure in the water near the earth was much greater than
at a distance from it. This shows where van 't HofTs
argument fails. He has negleded the difference of pres-
sure in the solvent near and far from the salt, or at least



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150



Theory of Solution.



I March a6, 189X.



has assumed that this difference of pressure could not adt
upon his semi-permeable membrane because the mem-
brane is permeable to the solvent. It is, however, quite
evident that the water can press very hard even on a
membrane permeable to it, as is explained by the example
I have just mentioned. Considering the complex nature
of the problem, I think it is quite too soon to assume that
the state of affairs assumed by van 't Hoff is at all like
reality. I would much rather look for an explanation in
the diredion I have pointed out-in this year's report of
the Committee on Eledrolysis. The argument there
tends to show that the distances between molecules would
arrange themselves so that the forces due to different
kinds of molecules would be independent of their kind and
depend on their numbers, and this would lead to the laws
of osmotic pressure. It seems to me much more likely
that a state of affairs such as I have supposed existing
near the earth is the one existing in a liquid.

As regards the argument for the independent mobility
of the ions founded on the laws of eledrolysi^, I think
that just as in the case of osmotic pressure this does
require a certain kind of independent mobility, but just as
in that case I do not see that the required amount of
independence calinot be attained without supposing a
complete independence. There seems no doubt that con-
dudlivity and double composition are essentially con-
neded with the same quality in the solution, and this
property I have proposed to call *' ionisation.** Now,
Williamson's hypothesis as to the nature of double
decomposition and Clausius's as to the nature of eledro-
lytic conduAion only require that the ions shall be so far
free as that they shall be frequently exchanging partners ;
neither, hypothesis nquires that they shall be during a
finite time without partners, which I consider to be an
essential condition of any right use of the term disso-
ciated ions. If during the time the ions are paired they
can move independently within the little chinks they have
to move in between the molecules of the solvent — and be
it observed that this is the same condition as for the extra
osmotic pressure, i.«., if their chinks are small compared
with the variation of force between the ions — then there
seems quite sufficient independence for any theory of
eledrolysis, if, whenever two molecules were within the
same chink, there were, as there would be, sufficient inde-
pendence for an exchange of partners. Thus these two
phenomena would be explicable upon the same hypothe-
sis, and that without assuming that ionisation was a true
dissociation. I have already explained that even those
who insist most strongly upon the dissociation hypothesis
yet guard themselves from its being supposed that this
dissociation is an aAually complete independence of the
ions from one another. On all these grounds then I
would appeal against the use of the word dissociation in
this connexion. ProfesFor Ostwald says that there will
result two theories leading to the same result. I dissent
from this. The two theories are essentially the same.
There have been unnecessary assumptions no doubt made
as to how far an absolute independence of motion of the
ions is required by the experiments, and I combat this
unnecessarily absolute independence, but in all essential
resped\s my theory is the same as the other. This un-
necessarily absolute independence has been introduced in
order to make what is acknowledged to be an ** analogy"
appear as if it were more than an analogy, to give veri-
similitude to what is at the same time said to be merely a
fagoH de parler, to make what is known to be complicated
appear unreally simple.

It may be worth while following Professor Armstrong's
suggestions that the way in which the double decomposi-



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